When I try to run Metacity in Ubuntu2D(12.04), I get the following message:

alisa@ubuntu:~$ metacity 
Window manager warning: Screen 0 on display ":2.0" already has a window manager; 
try using the --replace option to replace the current window manager.

I get the same message when running Compiz from the command line in 3D (it opens fine through the GUI (same thing for AWN)). I understand that these should be the default managers for the respective sessions.

I'm trying to create a custom Gnome session using the following instructions: unity launcher-free session. Here is what I've put into my .session file:

[GNOME Session]
Name=Custom Unity2D Session
RequiredComponents=gnome-settings-daemon;
RequiredProviders=windowmanager;panel;
DefaultProvider-windowmanager=metacity
DefaultProvider-panel=unity-2d-panel
FallbackSession=ubuntu-2d
DesktopName=GNOME

Since I'm having problems identifying my default, and the code refers to Metacity, I figured this may be relevant to my inability to load the custom session (it shows up on my login screen, but won't launch). I tried specifying Metacity as my default manager by adding exec metacity to the .xinitrc file, and I tried running metacity --replace, but neither worked.

How do I determine my current default window manager, what should the default be, and how do I re-assign it? Also, please let me know if you think there may be other issues affecting my custom session.

I am new to Linux, so list anything you think might be helpful. Thank you!